Highly efficient and robust molecular ruthenium catalysts for water oxidation

Abstract

Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H 2 driven by solar radiation (H 2 O þ hν → 1∕2O 2 þ H 2 ). The oxidation of water (H 2 O → 1∕2O 2 þ 2H þ þ 2e − ) provides protons and electrons for the production of dihydrogen (2H þ þ 2e − → H 2 ), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H 2 bda ¼ 2,2 0 -bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes ½RuðbdaÞL 2 (L ¼ pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce IV -driven ½Ce IV ¼ CeðNH 4 Þ 2 ðNO 3 Þ 6 water oxidation with high oxygen production rates up to 286 s −1 and high turnover numbers up to 55,400.catalysis | density function theory | seven coordination | photosystem II | solar fuels I n pursuit of sustainable energy systems such as solar fuels, much effort has been spent on water splitting to hydrogen and oxygen since hydrogen is a potential clean energy carrier and water is an abundant and environmentally benign resource (1-5). Water splitting consists of two half reactions: (i) water oxidationIn practice, an overpotential is always present, leading to an even higher applied potential. The former half reaction requires strongly oxidizing conditions and is generally considered as the bottleneck of the whole water-splitting process due to the multiple protonelectron transfers and the formation of the O─O bond. Over the last few years, there has been an increasing development of water oxidation catalysts (WOCs) and many transition metal-based catalysts, including Ru (4, 5), Ir (6-8), Co (9-13), Fe (14,15), and Mn (16-18), have been reported with oxygen production rates (OPRs: mole oxygen produced per mole catalyst per second) ≤5 s −1 . Very recently, we reported a family of highly active Ru-based WOCs ½RuðbdaÞL 2 (H 2 L ¼ 2;2 -bipyridine-6,6′-dicarboxylic acid; L ¼ 4-picoline, A; L ¼ isoquinoline, B) ( Fig. 1) with OPRs up to 300 s −1 (19, 20); a seven-coordinate dimeric Ru IV intermediate (D7Ru IV ) (Fig. 1) is involved in the O─O bond formation step (20,21).A general problem encountered in molecular WOCs is the decomposition of catalysts. Ligand dissociation and oxidative decomposition have been considered as the major deactivation pathways. The groups of Llobet, Meyer, and Sun have demonstrated an improved durability of their catalysts by immobilizing catalysts on the electrode/material surface and thereby dramatically suppressing the intermolecular oxidative decomposition pathway (22-25).For our Ru-bda catalysts, the main deactivation pathway has been found to b...

Electrochemical and Spectroscopic Study of Mononuclear Ruthenium Water Oxidation Catalysts: A Combined Experimental and Theoretical Investigation

“…Water oxidation catalysts (WOCs) are a key component of the artificial photosynthetic system. [4][5][6][7][8][9][10][11][12][13][14] Initially, it was supposed that only multinuclear complexes would be able to supply the oxidation potential needed to catalyse water splitting, in analogy with the four Mn complex in the oxygen evolving centre of Photosystem II. 8,15 As a result, several molecular catalysts have been developed with two or more metal sites.…”

“…2,8,11,[16][17][18][19] Recently, a shift of attention has occurred, directing more effort towards developing mononuclear WOCs. 8,20 A large number of ruthenium [4][5][6][7][8][9]13,[21][22][23] and iridium 8,11,14,[24][25][26][27][28][29] based catalysts have been reported. Density Functional Theory (DFT) has been used extensively to predict the reaction mechanisms and the electronic properties of several multi-and mono-nuclear molecular catalysts.…”

Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

“…Sun and coworkers (21,22) have described the Ru single-site water oxidation catalysts, [Ru II (bda)(L) 2 ] (H 2 bda is 2,2′-bipyridine-6,6′-dicarboxylic acid, HCO 2 -bpy-CO 2 H; L is isoquinoline, 4-picoline, or phthalazine). They undergo rapid and sustained water oxidation catalysis with added Ce IV .…”

Introducing a closed system approach for the investigation of chemical steps involving proton and electron transfer; as illustrated by a copper-based water oxidation catalyst

“…[1][2][3][4] A key component of the artificial leaf is the water oxidation catalyst, the optimisation of which is an active field of research especially using abundant transition metals such as copper and iron. [5][6][7][8][9][10][11][12][13][14][15][16][17] One way of optimising a proposed water oxidation catalyst is to consider its catalytic cycle and ensuring that it is as favourable as possible with low overpotential. 18,19 Traditionally, catalytic cycles are examined computationally by comparing the free energies of the proposed catalytic intermediates and then, usually, assigning the thermodynamically most favourable cycle as the most likely catalytic cycle.…”